1. Field of the Invention
The present invention relates to a light signal switching apparatus that has a carrier-injected optical waveguide type optical switch used in the optical communication system or the measuring apparatus for the optical communication therein and switches the light signal at a high speed and more particularly, a light signal switching apparatus capable of reducing the polarization dependency.
2. Description of the Related Art
In the related art, the optical switch element serving as the 2-input/2-output optical switch, which changes the refractive index by injecting the current (carriers) into the current injection region provided to the semiconductor optical waveguide structure, was manufactured. Such optical switch element is called the carrier-injected optical waveguide type optical switch.
FIG. 5 and FIG. 6 are a plan view and a sectional view showing an example of the carrier-injected optical waveguide type optical switch in the related art respectively (see Document 1, for example). In FIG. 5, an “X-shaped” optical waveguide 2 is formed on a substrate 1. An electrode 3 is formed in an intersection portion of the “X-shaped” optical waveguide 2. An electrode 4 is formed in close vicinity to the intersection portion of the “X-shaped” optical waveguide 2 in parallel with the electrode 3.
Meanwhile, FIG. 6 is a sectional view-taken along an “A-A′” line in FIG. 5. In FIG. 6, a substrate 5 is made of p-type Si, or the like. A core layer 6 is formed of a p-type SiGe layer, and is formed on the substrate 5. Then, most of the incident lights are guided by this core layer 6 to propagate through there. Also, the “X-shaped” optical waveguide 2 is formed in the core layer 6. A contact n+-region 7 is formed in the intersection portion of the optical waveguide 2. A contact p+-region 8 is formed in close vicinity to the intersection portion.
An insulating film 11 is made of SiO2, or the like, and is formed on the core layer 6 except upper surfaces of the n+-region 7 and the p+-region 8. An n-side electrode 9 is formed on the n+-region 7. A p-side electrode 10 is formed on the p+-region 8.
Next, an operation of the related example shown in FIG. 5 and FIG. 6 will be explained hereunder.
When the optical switch is turned “OFF”, no electric current is supplied to the electrode 3 (electrode 9) and the electrode 4 (electrode 10). For this reason, no change of the refractive index is caused in the intersection portion of the “X-shaped” optical waveguide 2 shown in FIG. 5. Therefore, for example, the light signal that is incident from an incident end Pi goes straight through the intersection portion, and then is output from an emergent end P1.
In contrast, when the optical switch is turned “ON”, an electric current is supplied from the p-side electrode 10 to the n-side electrode 9 via the n+-region 7. That is, the electrons are injected from the electrode 3 (electrode 9) and the holes are injected from the electrode 4 (electrode 10). Therefore, the carriers (electrons, holes) are injected into the intersection portion. Then, a carrier density is increased in the optical waveguide located near the n+-region 7 side.
According to this increase of the carrier density, the refractive index in the intersection portion of the “X-shaped” optical waveguide 2 shown in FIG. 5 is changed to lower. For example, the light signal incident on the incident end Pi is totally reflected by a boundary between a low reflective index area generated in the intersection portion (an area of the optical waveguide 2 close to the n+-region 7 side) and an area in which a change of the reflective index is seldom generated (a remaining half area of the optical waveguide 2), and then is output from an emergent end P2.
As a result, if the carriers (electrons, holes) are injected into the intersection portion of the “X-shaped” optical waveguide 2 (optical switch portion) by supplying the electric current to the electrode to control the reflective index in the intersection portion, it is feasible to control the position from which the light signal is output, in other words, switch the transmission path through which the light signal propagates.
Therefore, if a light reflection is generated in a condition that the boundary between the area, in which the change of the refractive index is generated because the carrier density is increased by the current injection, and the area, in which the change of the refractive index is not generated, is made definite in the optical waveguide 2, the light reflection area can be produced effectively.
Also, the change of the refractive index due to the carrier density is generated based on the plasma dispersion effect or the band filling effect (see Document 2 and Document 3, for example). Therefore, the change of the refractive index is increased at the same carrier density if effective masses of carriers (a free electron and a free hole) are small. For this reason, if the material system in which the effective mass is smaller is used, the large change of the refractive index can be generated by a smaller amount of current injection (smaller current density), and thus the optical switch that can be driven by the small current can be realized.
Next, an example of a sectional view of the related optical switch using the material system in which the effective masses of the carriers (the free electron and the free hole) is small (e.g., InP-based material) is shown in FIG. 7 (see Document 4, for example).
In FIG. 7, a substrate 12 is made of InP, or the like. A core layer 13 is formed of an n-type InGaAsP layer as a quaternary layer, for example, and is formed on the substrate 12. An n-type InP layer 14 is formed on the core layer 13. An n-type InGaAsP layer 15 is formed on the n-type InP layer 14. An insulating film 16 is made of SiO2, or the like, and is formed on the InGaAsP layer 15. A p-side electrode 17 is formed on the insulating film 16, and an n-side electrode 18 is formed on the back surface of the substrate 12.
In the optical switch shown in FIG. 7, the core layer 13, the InP layer 14, and the InGaAsP layer 15 are formed sequentially on the substrate 12, and also the “X-shaped” optical waveguide is formed by etching respective layers up to the core layer 13.
Then, the p-type impurity is diffused into a portion indicated by “DR11” in FIG. 7. Then, the electrode 17 is formed to come into contact with the portion indicated by “DR11” in FIG. 7, and the electrode 18 is formed on the back surface.
In the related art shown in FIG. 7, since the material system in which the effective masses of the carriers (the free electron and the free hole) is small is used, the large change of the refractive index can be generated by a smaller amount of current injection (smaller current density), and thus the optical switch that can be driven by the small current can be realized.
Then, an optical switch having such a structure that limits the refractive-index changing area by using the current constriction will be explained hereunder. FIG. 8 and FIG. 9 are a plan view and a sectional view showing an example of the related optical switch, in which the current constriction is applied by providing a p-type region to the optical waveguide in the intersection portion and thus the refractive-index changing area is limited by limiting the high carrier density area, respectively (see Document 5, for example) Also, a configuration of the optical switch in the case where the optical switch is used actually as a light signal switching apparatus is shown in FIG. 4.
In FIG. 8, an “X-shaped” optical waveguide 20 is formed on a semiconductor substrate 19. An electrode 21 is formed in an intersection portion of the “X-shaped” optical waveguide 20. Also, the light signal is transmitted through an input optical fiber Fi, and then is incident on an incident end (e.g., Pi′) of the optical switch by a converging lens system Li. Also, the light signal being emergent from an emergent end P1′, P2′ of the optical switch is incident on an output optical fiber Fo by a converging lens system Lo and is transmitted through there. In this case, both the input optical fiber Fi and the output optical fiber Fo consist of a single mode optical fiber respectively.
Meanwhile, FIG. 9 is a sectional view taken along a “B-B′” line in FIG. 8. In FIG. 9, a substrate 22 is made of InP, or the like, for example. A lower cladding layer 23 is formed of an n-type InP layer, for example, and is formed on the substrate 22. A core layer 24 is formed of an n-type InGaAsP layer, and is formed on the lower cladding layer 23. An upper cladding layer 25 is formed of an n-type InP layer, and is formed on the core layer 24. A contact layer 26 is formed of an n-type InGaAsP layer, and is formed on the upper cladding layer 25.
In FIG. 9, Zn as the p-type impurity is diffused into portions indicated by “DR31” to “DR33”. An oxide layer 27 is made of SiO2, or the like, and is formed on the contact layer 26 in the area except the diffusion area indicated by “DR33” in FIG. 9. A p-side electrode 28 is formed on the diffusion area indicated by “DR33” in FIG. 9. An n-side electrode 29 is formed on the back surface of the substrate 22.
Next, an operation of the related art shown in FIG. 8 and FIG. 9 will be explained hereunder. When the optical switch is turned “OFF”, no electric current is supplied to the electrode 21 (electrode 28) and the electrode (not shown) formed on the back surface of the substrate 19 (corresponding to the electrode 29 in FIG. 9).
For this reason, no change of the refractive index is caused in the intersection portion of the “X-shaped” optical waveguide 20. Therefore, for example, the light signal that is incident on the incident end Pi′ from the input optical fiber Fi and the converging lens system Li goes straight through the intersection portion, and then is output from the portion indicated by the emergent end P1′. Then, the light signal is transmitted through the output optical fiber Fo via the converging lens system Lo.
In contrast, when the optical switch is turned “ON”, an electric current is supplied to the electrode 21 (electrode 28) and the electrode (not shown) formed on the back surface of the substrate 19 (electrode 29). Thus, the carriers (electrons, holes) are injected into the intersection portion.
Therefore, the refractive index of the intersection portion of the “X-shaped” optical waveguide 20 directly under the electrode 21 is changed by the plasma effect to become low. Thus, the light signal being input from the incident end Pi′ is totally reflected by the boundary to the low reflective index portion produced in the intersection portion and emitted from the portion indicated by the emergent end P2′. Then, the light signal is transmitted through output optical fiber Fo via the converging lens system Lo.
As a result, if the carriers (electrons, holes) are injected into the intersection portion of the “X-shaped” optical waveguide 20 (optical switch portion) by supplying the electric current to the electrode to control the reflective index in the intersection portion, it is possible to control the position from which the light signal is output, in other words, switch the transmission path through which the light signal propagates.
[Document 1] Baujun Li, Guozheng Li, Enke Liu, Zuimin Jiang, Chengwen Pei and Xun Wang, “1.55 μm Reflection-type Optical Waveguide Switch based on SiGe/Si Plasma Dispersion Effect”, Appl. Phys. Lett., Vol. 75, No. 1, pp. 1-3, (1999)
[Document 2] The Japan Society of Applied Physics (JSAP), Editors of Social Meeting for Optics Discussion, “Optical Integrated Circuit-Fundamentals and Application”, first edition, Asakura Publishing Co., Ltd., Apr. 10, 1988, Chapter 5, p. 104
[Document 3] Baujun Li and Soo-Jin Chua, “2×2 Optical Waveguide Switch with Bow-Tie Electrode Based on Carrier-Injection Total Internal Reflection in SiGe Alloy”, IEEE Photon Tech. Lett. Vol. 13, No. 3, pp. 206-208, 13 (2001)
[Document 4] Hiroaki Inoue, Hitoshi Nakamura, Kenichi Morosawa, Yoshimitsu Sasaki, Toshio Katsuyama, and Naoki Chinone, “An 8 mm Length Nonblocking 4×4 Optical Switch Array”, IEEE Journal of Selected Areas in Communications, Vol. 6, No. 7, pp. 1262-1266, (1988)
[Document 5] K. Ishida, H. Nakamura, H. Matsumura, T. Kadoi, and H. Inoue, “InGaAsP/InP Optical Switches using Carrier Induced Refractive Index Change”, Appl. Plys. Lett., Vol. 50, No. 19, pp. 141-142, (1987)
As shown in the related art in FIG. 5 and FIG. 8, the related optical switching element executes the optical switching operation by utilizing the reflection that is generated due to the reflective index change in the optical switch portion caused by the carrier injection. Also, the light incident on the incident end Pi (or the incident end Pi′) is switched from the emergent end P1 to the emergent end P2 (or the emergent end P1′ to the emergent end P2′), for example. Also, as shown in FIG. 8, the light signal switching apparatus is employed on the premise that the optical fibers Fi, Fo used to input/output the light are connected to the optical switch single body via the converging lens systems Li, Lo respectively. Therefore, improvement of the characteristics of the optical switching element can be achieved.
However, in case the light signal switching apparatus is constructed in this manner, an optical power intensity (an extinction ratio and an insertion loss of respective outputs) derived when the light incident on the incident end Pi, Pi′ is switched to the transmission-side emergent end P1, P1′ or the reflection-side emergent end P2, P2′ depends upon the polarization of the incident light. The element structures containing the optical waveguide structure of the optical switch element, the structure of the optical switch portion, and the profile of the reflective-index changing area by the carrier injection exert an influence complexly upon this polarization dependency.
Also, a ratio of the optical power intensities that are switched to the emergent end P1 and the emergent end P2, the emergent end P1′ and the emergent end P2′ is changed by the structure of the optical switch portion (the reflective-index changing area, an intersection angle of the optical waveguides, etc.), a width of the optical waveguide, and the like. And, normally the optical power intensity on the transmission side tends to increase.
Also, there existed the problem, as long as the light signal switching apparatus employs the structure adopted in the related art (structure in which the optical fibers Fi, Fo used to input/output the light are connected to the optical switch single body via the converging lens systems Li, Lo respectively), it is difficult for such light signal switching apparatus to overcome these subjects.
In particular, the light signal incident on the light signal switching apparatus is transmitted over the optical fiber having a length of several tens [m] to several [km] or more. Therefore, the polarization of the transmitted light signal is varied in time depending on variations in the environments (for example, pressure, temperature, magnetic field, etc.) in which the optical fiber is laid down. As a result, in the case where the light signal switching apparatus is used actually, there existed the problem, in the configuration of the light signal switching apparatus using the optical switch element shown in the related art, an output intensity of the light being switched by the optical switch portion is changed according to the polarized condition of the incident light.
In contrast, in the case where the subject about the polarization dependency in the light signal switching apparatus should be avoided by using the polarization maintaining optical fiber that preserves the polarized condition, the waveform of the light signal is deteriorated because normally the propagation light is affected largely by PMD (Polarization Mode Dispersion) in the light signal switching apparatus. Therefore, it was difficult to transmit the light signal over the long distance at an optical bit rate in the [Gbps] band.